1. Field of the Invention
The present invention relates to injection-molded, rigid and etched polymeric substrates for optical recording or reading discs, based on thermotropic polymers This invention also relates to the optical discs fabricated from such substrates
By the term "optical disc" as utilized herein, there are intended all types of recording media for readout by a laser beam, which may be recorded:
(a) either during their manufacture, by a molding or pressing stage which forms a microrelief on the surface of the disc substrate (compact disc or video disc type);
(b) or by means of an irreversible thermal inscription, also produced under a laser beam and generally characterized by a modification of the sensitive recording layer in the form of holes or bubbles or other local microdamage;
(c) or in a reversible thermal or thermomagnetic manner, without the formation of an irreversible physical modification of the sensitive layer, by producing a change of state or in the direction of magnetization of the layer which, by virtue of its reversibility, presents the option of a deletion or a rewriting.
2. Description of the Prior Art
In order to produce an optical disc, materials of three types are generally combined, in a manner known per se (see FIGS. 1 and 2 of the attached drawings, each of which shows, on no particular scale, a view of an optical disc in fragmentary section through a plane containing the axis of the disc):
(i) a substrate 1 which is in most cases a transparent material and consequently serves as an input layer for the luminous beam. The thickness of this input layer generally ranges from 1 mm to 2 mm. To enable the recording track to be defined and to be traced by the laser beam, part or all of the face surface distant from the input face surface of this substrate 1 is provided with grooves (or reliefs) 2 of concentric shape, or arranged in a spiral, inside which or between which are stored the data required for the synchronization, data identification or tracking control. It is highly advantageous to produce the microreliefs 2 directly at the time when the substrate 1 is molded, but this is possible only in the case of materials capable of being injection-molded. In the case of a material such as glass, this microrelief must be produced by a stage of photopolymerization of a varnish deposited in the form of a very thin layer. This operation must be carried out under very clean conditions to avoid defects due to contamination by dust. It can also be the source of defects which can extend to the rejection of the disc because of the formation of microbubbles or of a tearing away of the microrelief over the course of the molding and demolding utilizing a die member. For these reasons, it is preferable to have the option of employing a polymeric material as the substrate 1 and to produce the microrelief 2 directly at the point in time of the injection molding;
(ii) a reflective sensitive layer 3 secured to the said pre-etched substrate (in the case where the disc already contains all the data, this layer obviously serves only as the reflective layer); and
(iii) a rear protective or backing layer 4, transparent or opaque, which may be defined simply by a protective varnish. In the case of a disc having a double input face surface, this rear layer may be a structure which is identical to that of the input layer 1.
The assembly of the input layer 1 and of the rear layer 4 is via connecting means 5 which may, for example, consist of resilient coaxial seals 6 which ensure perfect leakproofing if desired (the disc then has a structure known as the air seal sandwich type 7 or of a transparent bonding material 8 (the disc then has a structure known as the laminated type).
In the case of a disc having a single input face, such as that described in FIGS. 1 and 2, it is quite advantageous to employ the same type of transparent material to manufacture both the input layer 1 and the protective layer 4.
In one known alternative embodiment (see FIG. 3 of the drawings), the sensitive reflective layer 3 may also be disposed on the face surface of the rear layer 4 which confronts the input layer 1; in this embodiment, the rear layer 4 then serves as a substrate, while the input layer 1 serves only as an optical separation. A structure of such type permits a nontransparent material to be employed as substrate 4 if need be. U.S. Pat. No. 4,074,282 describes a disc having a structure of this type, in which the thickness of the substrate 4 has been reinforced and that of the input layer 1 has been reduced in order to make it possible to use materials of low rigidity to produce the substrate.
In contradistinction to the case of the structures of FIGS. 1 and 2, in the case of the structure of the type as that shown in FIG. 3, the substrate 4 whereon the microetching 2 and the sensitive layer 3 are deposited no longer serves as an input receiving layer, but, to the contrary, serves as a rear layer. In this case, the material of the input layer 1 must be perfectly planar and transparent.
However, the structures of the type as shown in FIG. 3 are of no particular interest when the material employed as the substrate 4 presents no outstanding rigidity or stability properties while remaining capable of being injection-molded.
Indeed, in the case of a glass substrate, the structures which are generally preferred are those of the types of FIG. 1 or 2, a defective substrate being reemployed, where applicable, for the rear layer 4. However, glass substrates are particularly costly to manufacture and mandate an additional photopolymerization operation which, moreover, affects the efficiency of manufacture. Attempts have therefore been made to replace the glass with injection-molded plastic substrates.
To date, attempts have been made to utilize transparent materials for the manufacture of plastic substrates, such as to produce structures of the types shown in FIG. 1 or 2, in which the substrate also serves as an input layer.
The fact that a polymeric substrate is required to have excellent optical properties (high transparency, absence of birefringence, homogeneity) limits the selection of polymeric materials as a practical matter, to materials of amorphous structure, because it is known that semicrystalline polymers, which incidentally have the advantage of superior mechanical and thermal properties, are opaque or highly light scattering materials. It is known to this art, for example, that polymethylmethacrylate and even polycarbonate are materials which are potentially too limited, with regard to rigidity and optical properties, to be employed in optical discs which comprise moisture- or oxidation-sensitive recording layers.
The injection-moldable polymeric material adapted for use as a substrate must combine very diverse properties such as:
(1) Excellent moldability (fluid material) to permit the filling of mold microcavities and shrinkage-free demolding;
(2) The feasibility of producing undistorted planar articles (absence of internal stresses liable to produce buckling)
(3) A high rigidity which is required for the stability of the assembled disc, especially to prevent the distortion of the sensitive layer under the influence of external pressure changes; and
(4) High dimensional stability (thermal behavior, insensitivity to moisture, low coefficient of expansion).
Also to date, transparent plastic substrates have been successfully employed only for discs of small size and when the recording layer is not highly oxidation- or moisture-sensitive. However, recent work has demonstrated the need to investigate materials of other types, particularly modified polyolefins and modified polycarbonate, but this is an extremely limited direction of improvement, insofar as certain required properties (for example, high rigidity and transparency) cannot be concomitantly imparted.
The addition of fillers is a known technique for correcting the natural values of polymeric materials with regard to rigidity (when it is too low; for example, when a modulus o elasticity is too low) and dimensional stability (when it also is too low; for example, when a coefficient of heat expansion is too high). However, the use of fillers must be avoided in molding rigid substrates made of a plastic material for optical discs, since their presence can play a role which is detrimental to surface quality and to dimensional stability. Furthermore, the melt viscosity of filled polymer is too high to permit precision molding with good reproducibility of the die corresponding to the microrelief, as discussed above.